1. Field of the Invention
The present invention relates to a power supply device, and more particularly to a power supply device of a high output voltage adapted for use in an image forming apparatus, such as a copying apparatus or a printer, utilizing the electrophotographic process.
2. Related Background Art
FIGS. 5 and 6 are circuit diagrams showing different examples of conventional power supply devices.
In an example shown in FIG. 5, there are provided a high-voltage transformer 181; transistors 182, 183, 184; an operational amplifier 185; drivers 186, 187 respectively for the transistors 182,183; load current detecting means 189; and a control circuit 188 for supplying the operational amplifier 185 with a level signal and the drivers 186, 187 with clock signals of inverted phases, in response to a signal from the current detecting means 189. There are also provided high-voltage diodes 190,191; high-voltage capacitors 192, 193; a load 194; and a high-voltage relay 195.
The conventional example mentioned above functions in the following manner. This circuit effects constant-current control on high output voltages of positive and negative polarities. When the transistor 183 is turned on, a positive high voltage is generated at the secondary side of the transformer 181 to turn on the high-voltage diode 191, thereby charging the high-voltage capacitor 193 with a positive high voltage. On the other hand, when the transistor 182 is turned on, a negative high voltage is generated at the secondary side of the transformer 181 to turn on the high-voltage diode 190, thereby charging the high-voltage capacitor 192 with a negative high voltage.
The polarity of the high voltage applied to the load 194 is switched by the high-voltage relay 195. Then, depending on the signal from the current detecting means 189, the control means 188 varies the signal supplied to the operational amplifier 185, thus regulating the voltage applied to the transformer 181 through the transistor 184. Thus the current in the load 194 is controlled at a predetermined value in this manner.
In the circuit diagram in FIG. 6, showing another conventional example, there are provided high-voltage transformers 201, 202; transistors 203, 204; oscillators 205, 206; control means 207 for terminating the oscillation of the oscillator 205 or 206 according to the polarity of the high output voltage; high-voltage diodes 208, 209; high-voltage capacitors 210, 211; resistors 212, 213; a load 214; and diodes 220, 221.
Referring to FIG. 6, the transformers 201, 202 respectively generate positive and negative high voltages. The polarity of the high voltage supplied to the load 214 is switched according to whether the control means 207 causes oscillation in the oscillator 205 or 206.
The conventional example shown in FIG. 5 requires the expensive high-voltage relay 195 for switching the polarity of the output high voltage, thus leading to a significantly elevated cost. Also if there are required different voltage levels at the positive and negative sides, there are required not only the high-voltage relay but also plural units of expensive high-voltage transformers. Consequently such structure is unfavorable not only in terms of the cost but also in terms of the space required.
On the other hand, the conventional example shown in FIG. 6 does not require the high-voltage relay for the polarity switching, but always requires plural high-voltage transformers even in case of generating positive and negative high voltages of a same magnitude. Consequently the structure becomes complex and bulky, leading again to an elevated cost.